This invention relates to thin film solar cells. More specifically, this invention relates to thin film hydrogenated amorphous silicon solar cells.
Photovoltaic cells, such as hydrogenated amorphous silicon solar cells, are capable of converting solar radiation into usable electrical energy. The electrical energy conversion occurs as a result of what is well known in the solar cell field as the photovoltaic effect. Solar radiation impinging on a solar cell is absorbed by the semiconductor layer, which generates electrons and holes. The electrons and holes are separated by a built-in electric field, for example, a rectifying junction, such as an N-I-P junction, in the solar cell. The electrons flow toward the N-type region and the holes flow toward the P-type region. The separation of the holes and electrons across the rectifying junction results in the generation of an electric current known as the photocurrent and an electric voltage known as the photovoltage.
Photovoltaic researchers have been investigating various paths toward the generation of electricity from sunlight which can compete on an economic basis with conventional means of generating electricity. One of the main areas of focus for research is low cost, large area hydrogenated amorphous silicon solar cells. Hydrogenated amorphous silicon is a low cost semiconductor material which can be manufactured by processes such as glow discharge, sputtering, or reactive sputter deposition. These processes are adaptable to automated manufacturing techniques and large area panels. Current research focuses on improving the efficiency of the devices as the area of the cells increase. The cells can be fabricated in many known semiconductor configurations, such as P-I-N solar cells, N-I-P solar cells, Schottky barrier solar cells, and MIS solar cells (the first letter designates the incident surface through which solar radiation first penetrates the cell).
Thus far, devices incorporating a photoactive intrinsic region, such as NIP devices and the like, have exhibited the highest efficiencies. However, Schwartz teaches, in J. Appl. Phys. Vol. 53, 715 (1982), that a limiting factor in an NIP solar cell is the back diffusion of holes into the N-type layer which lowers the short circuit current (J.sub.sc) of the device. In addition, the photoactive intrinsic region of the NIP device has a bandgap greater than the optimum bandgap for the maximum absorption of solar radiation. Thus, it would be highly desirable to have an NIP solar cell wherein solar radiation enters through the N-type region, which minimizes the back diffusion of holes, and has a photoactive intrinsic region more closely matched to the ideal bandgap for increased absorption of solar radiation.